Intrusion detection apparatus employing a pressure-differential detector

ABSTRACT

An intrusion detection apparatus employs a pressure differential generating means, such as a blower of fan, and a pressure differential sensor which senses changes in pressure from a steady state condition which occur upon the opening of doors or windows. A highly sensitive balanced and gravity biased vane is mounted in a chamber formed for communication between the volume enclosed by the structure and the exterior thereof to provide a sensor enabling an accurate, reproduceable sensing of small changes in openings to the exterior without being triggered by movement within the structure. A photoelectric vane detection means is employed to determine vane position without physically contacting the same. Means for adjusting the response sensitivity of the vane and an armed-before-alarm circuit are also disclosed.

United States Patent 1191 Evans et al.

[ 5.] Aug. 13, 1974 both of Hayward; Rodney R. Ward, Castro Valley, all of Calif.

} [73] Assignee: William H. Clark, III, Lafayette,

Primary Examiner-John W. Caldwell Assistant Examiner-Glen R. Swann Attorney, Agent, or FirmWarren, Rubin & Chickering [57] ABSTRACT An intrusion detection apparatus employs a pressure Califdifferential generating means, such as a blower of fan, [22] Filed: June 21, 1973 and a pressure differential sensor which senses changes in pressure from a steady state condition PP NOJ 372,227 which occur upon the opening of doors or windows. A highly sensitive balanced and gravity biased vane is 52 U.S. c1.....- 340/258 R, 340/240 340/276 mounted in a Chamber formed for Communication 200/819 tween the volume enclosed by the structure and the [51] Int. 01. G08b 13/20 exterior thereof to Provide a Sensor enabling accu- [58] Field of Search 340/258 D 258 R 240 reproduceable Sensing of Small Changes in Open 340/239 R 274 200/2519 R 5 ings to the exterior without being triggered by movement within the structure. A photoelectric vane detection means is employed to determine vane position [56] References Cited without physically contacting the same. Means for adjusting the response sensitivity of the vane and an UNITED STATES PATENTS armed-before-alarm circuit are also disclosed.

1,025,310 5/1912 Robillot 340/276 4 F. 2,559,402 7/1951 Comstock. 73/228 14 Clam, Drawmg 3,289,192 ll/1966 Davey 340/276 L2 7 .22 /O i ll D-E /7& ii

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FIG. 2

INTRUSION DETECTION APPARATUS EMPLOYING A PRESSURE-DIFFERENTIAL DETECTOR BACKGROUND OF THE INVENTION The rising crime rate and the associated concern of the average American has resulted in a rapidly increasing need for an inexpensive intrusion detection system operable in the average home, store or factory. Any intrusion detection apparatus which is to meet this need must be easily installed, simple to operate, and protect against intrusions at any point of a small or large volume.

Since any intrusion into an enclosed volume involves the establishing of an opening therein and since an opening in the enclosed volume will dissipate any pressure differential between the enclosed volume and the outside, intrusion detection apparatus have previously been devised to sense and detect pressure drops resulting from an unwanted entry. A typical device of this type is disclosed in U.S. Pat. No. 3,289,192 in which a blower is used to set up a pressure differential between an enclosed volume and the outside, and a detector unit, comprising a heated thermister element, is located in the air flow caused by the pressure differential. Connected to a bridge circuit is a similar thermistor which is located out of the air flow and employed as the temperature reference to enable measurement of changes in flow. Numerous other types of sensing means have been employed including those operating with a pressure sensitive diaphragm (U.S. Pat. No. 2,745,089) and those utilizing a vane which moves in response to air flow caused by the pressure differential (Swiss Pat. No. 189,772). These devices, however, lack the desired sensor sensitivity and require an undesirably high capacity fan or blower for accurate operation. Additionally, some are undesirably costly to construct and inherently subject to certain types of malfunctions. The vane-type sensors have required mechanical apparatus connected to or touching the vane to sense a change in its position in response to the air flow past it. This mechanical contact is required to trigger the electrical alarm, but it interferes substantially with the sensitivity of the unit.

One of the most sensitive measurement devices known to man is the balanced scale. The present invention seeks to utilize the advantages of the balanced scale by providing a balanced vane sensing means which is responsive to very small changes in pressure and resulting air flow. An accurate and highly sensitive sensing means in turn allows a relatively low capacity blower to be employed.

Accordingly, it is an object of the present invention to provide an intrusion detection apparatus which is highly sensitive to very small changes in pressure differential and which is relatively uninhibited in its response to air flow caused by the pressure differential.

It is another object of the present invention to provide an intrusion detection apparatus of the type above-described which is operable under a very small pressure differential.

It is a further object of the present invention to provide an intrusion detection apparatus of the type above-described which is adjustable in terms of sensitivity as a function of the character of the enclosure.

Still another object of the present invention is to provide an intrusion detection apparatus of the type above-described which is relatively insensitive to changes in the ambient temperature or humidity of the enclosed volume and external meteorological phenomena.

Yet another object of the present invention is to provide an intrusion detection apparatus of the type above-described in which the possibility of false alarms is minimized.

Still another object of the present invention is to provide an intrusion detection apparatus of the type above-described which is easy to manufacture, reliable, easy to install and operate, inexpensive, and adaptable to a wide variety of substantially enclosed environments.

The intrusion detection apparatus of the present invention has other objects and features of advantage which will become apparent from the drawings and are set forth in more detail in the description hereinafter.

SUMMARY OF THE INVENTION The intrusion detection apparatus of the present invention is of the type operating within a substantially enclosed volume and has pressure differential generating means mounted to communicate and formed to establish a pressure differential between the enclosed volume and an external volume and has pressure differential sensing means with improved sensitivity. Briefly, the improved sensing means is formed with a housing defining a chamber formed for communication of the pressure differential therethrough; means for mounting and positioning a vane in the housing; a vane pivotally mounted to and gravitationally balanced about the mounting means for movement to an armed position in response to said pressure differential and for gravity biasing to an alarm position under a condition of change in the pressure differential; and detecting means mounted to sense the presence of the vane in the alarm position. The vane is further formed with a first vane portion extending across the chamber to provide a movable wall responsive to the pressure differential for pivotal movement to the armed position and with a second vane portion positioned substantially outside the chamber. The chamber and a damping cavity in which the second vane portion moves preferably provide passageways at the ends of the vane of substantially constant dimension throughout the angular displacement of the vane between the armed and alarm position. Additionally, the detection means is preferably formed to determine the presence of the alarm position without physically contacting a movable portion of said vane. As a further convenience the invention incorporates adjustment means mounted to the housing and formed to enable selective adjustment of the response sensitivity of the vane. Additionally, alarm energizing circuitry is provided, constructed so that the alarm will normally be energized only when the vane has been first displaced to the armed position.

BRIEF DESCRIPI ION OF THE DRAWINGS FIG. 1 is a schematic diagram of a substantially enclosed three-room dwelling incorporating an intrusion detection apparatus of the present invention.

FIG. 2 is a side view in cross-section of a sensing means for an intrusion detection apparatus constructed in accordance with the present invention and taken along the plane of line 2-2 of FIG. 3.

FIG. 3 is a top view of the sensing means of FIG. 2.

FIG. 4 is a circuit diagram of the alarm energizing circuitry constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the intrusion detection apparatus of the present invention can be seen to include pressure differential generating means 11 and pressure differential sensing means 26. Generating means 11 may be a fan, blower, pump or the like and is positioned to communicate between and establish a pressure differential between a substantially enclosed volume, generally designated 12, and an external volume 13, in this case all volume external to three-room enclosure 10. (Although we here show the enclosure, or substantially enclosed volume, to be a dwelling, it is understood that the intrusion detection apparatus of the present invention may be advantageously employed in a plurality of enclosures including warehouses, vaults cargo containers, etc.) Enclosure includes access door 14, internal doors 16, 17 and 18 and windows 20, 21, 22, 23 and 24. Pressure differential sensing means 26 is mounted to wall 27 of the structure to sense the pressure differential and changes therein. While schematically illustrated as exhausting air from volume 12 of building 10 in a horizontal direction, generating means 11 is advantageously operated in a vertical orientation with respect to the external volume to reduce the effects of wind upon the desired maintenance of a constant pressure differential between enclosed volume 12 and external volume 13. For example, generating means 11 may operate through an access bore in the roof of enclosure 10 and may additionally operate through complementing rotating baffles of the type commonly found on roof vents which will assist in regulating the pressure differential. Additionally, generating means 11 may be oriented to pull air into structure 10 and create an increased pressure therein over the pressure of external volume 13.

Sensing means 26 (best seen in FIGS. 2 and 3) is formed with a housing 28 defining in part a chamber 29 formed for communication of a pressure differential between external volume 13 and enclosed volume 12, through bore 34 and opening 30 in housing 28. Housing 28 is preferably formed defining a substantially cylindrical cavity 31 having medially disposed partition 32 extending along the longitudinal axis of cavity 31 helping to define chamber 29 on one side thereof and on the other side thereof a damping cavity 33 (to be more fully explained below). Mounting means 36, in this case a rod or tube, is mounted relative to housing 28 and affixed to a vane generally designated 37 to position the vane within the housing. Thus, mounting means 36 may be rotatably received in housing bores 38 and 39 (FIG. 3) so as to pivotally mount vane 37 with a first vane portion 41 extending across and helping to define chamber 29. Vane portion 41 provides a movable wall responsive to the pressure differential between volumes l2 and 13. A second vane portion 42 is positioned by mounting means 36 substantially outside chamber 29 since partition 32 pneumatically insulates chamber 29 from cavity 33. It should also be noted that mounting means 36 can take the form of the upper surface of partition 32, which can seat in a mating recess in vane 37.

In order to provide an intrusion detection apparatus with greatly improved sensitivity to small changes in pressure differential, vane 37 is gravitationally balanced about mounting means 36 to provide a pressure sensitive device analogous to the balanced arm scale, so frequently used in the comparison or measurement of very small masses in a well-equipped scientific laboratory. The vane is responsive to a difference in pressure between chamber 29 and the remainder of enclosure 12; this pressure differential essentially reflects that established by generating means 11. In the embodiment shown in FIG. 2, vane 37 is shown in solid lines displaced to its armed position in response to a greater pressure in external volume 13 and chamber 29 than in enclosure 12, generating means 11 being assumed to lower the pressure in enclosure 12 with respect to external volume 13 by exhausting air therefrom as shown in FIG. 1. Vane 37 is urged to the armed position in a steady state mode under the normal operating pressure differential.

Under a condition of change in the pressure differential, in this case an increase of pressure in enclosed volume 12 by reason of the opening of a door or the like, vane 37 is mounted to rotate to the alarm position 46 (shown in broken lines) under gravity biasing. Vane 37 and mounting means 36 are formed to vertically offset the axis of rotation 43 of the vane-mounting means system from the center of gravity 44 of the system. Axis of rotation 43 is, in the preferred embodiment, the center of rod or tube 36 comprising the mounting means, whereas center of gravity 44 is vertically lower than axis of rotation 43 because of the mass distribution of the vane-mounting means system. When vane 37 is in alarm position 46, the center of gravity 44 of the vane and rod is directly under the center of rotation, causing the vane to be in a gravitationally balanced equilibrium position. Displacement of vane 37 in response to a pressure differential between chamber 29 and volume 12 causes center of gravity 44 to be laterally displaced from center of rotation 43. In this displaced position there is a resultant downward force at a laterally spaced distance from center 43 which effects gravity biasing of vane 37 from the armed position to position 46. In its normal operating condition shown in FIG. 2 this downward force is exactly offset by the force produced by the pressure differential acting over the area of the vane. Thus, any changes in the pressure differential will be reflected in the angular displacement of vane 37 in response thereto.

It should be noted that vane 37 can be readily weighted to have a balanced equilibrium position 46 by simply placing a weighting material on the vane portions. In practice, vane 37 is constructed of a sheet of lightweight material, such as aluminum, which is tackwelded or adhesively secured to rod 36. Drops of lacquer can be used to balance the vane to assume a steady equilibrium position at 46. Balancing vane portion 41 to an equilibrium position below a horizontal plane through mounting rod 36 with the armed position above a horizontal plane through the mounting means, insures that a minimum height of housing 28 is required for any selected angular rotation of vane 37 between armed and alarm positions.

The improved sensitivity of sensing means 26 derives partially from the balanced nature of vane 37 and the above-described gravity biasing thereof but is also due in large measure to the isolation between chamber 29 and cavity 33. This pneumatic isolation causes first vane portion 41 to be the only portion of the vane exposed to the pressure differential. In this fashion partition 32 prevents the pressure differential from acting on second vane portion 42, which would oppose the force produced over the area of portion 41 by the pressure differential and therefore reduce the sensitivity of the vane system with respect to the pressure differential.

Additionally, the sensitivity is also aided by the fact that vane 37 is uninhibited in its movement in response to changes in the pressure differential and has no mechanical linkage or contacts with vane detection means 47 and 48 (to be more fully explained below). Thus, neither the vane nor its mounting rod 36 are formed to physically touch or engage a sensing element to indicate the position of the vane. Therefore, there are no contact forces interfering with the inherently sensitive nature of the gravitationally biased balance vane system.

The vane has been found to be sensitive enough to operate under a very small steady state pressure differential and to respond to very small changes therein; pressure generating means 11 can be as small as a typical kitchen exhaust fan and provide a pressure differential within a multiple-room dwelling sufficient to operate sensing means 26. It is highly desirable to raise the sensitivity of sensing means 26 to the highest possible level causing a corresponding lowering of the capacity requirement of generating means 11 so that the enclosure will not be drafty and cold and so that power requirements for the generating means will be reduced.

In operation, sensing means 26 detects intrusions into substantially enclosed volume 12 as follows. First, structure is brought to a security condition, i.e., access to the interior is prevented by means of closing doors, windows, and other openings to external volume 13. It should be noted that structure 10 need not be airtight or even nearly so to create a substantially enclosed volume." The presence of chimneys, cracks under doors, and vents are quite acceptable. Even partially open windows will not affect the operation of improved sensing means 26, so long as the opening is not sufficient to permit the entry into enclosure 12 of the body of an intruder. Sensing means 26 is adjusted, as will be explained below, such that vane 37 comes to the armed position whenever the dwelling has been secured. As will be set forth in more detail, if enclosure 12 has occupants, they may conduct normal activities with in the enclosure, including the ability to gain ventilation from open windows while still being protected by the intrusion detection apparatus of the present invention.

Assuming that generating means 11 has established a partially evacuated condition in enclosure 12 with respect to external volume 13, an air flow through chamber 29 from bore 34, as a result of the pressure differential, will impinge upon vane 37 urging it upward to the position shown in FIG. 2 in solid lines, the armed position. Upon vane 37 coming to the anned position, detection means 47 and 48 assumes a condition which will activate an alarm if the vane is displaced back to alarm position 46, by means of circuitry more fully described below. Detection means 47 and 48 may take a variety of forms, the principal purpose of the detection means being to detect the presence of vane 37 in the alarm position without physically contacting a movable portion of the vane. In this case, the detection means comprising light emitting device 47 together with photosensor 48. The detection means need not necessarily be responsive only to visible light, but could be sensitive to infrared radiation equally advantageously; in fact, any source propagating an electromagnetic wave and a detection device sensitive to the wave being propagated will function advantageously. In the preferred embodiment the detection and reception of light at photosensor 48 signifies the armed position of vane 37, while interruption of the waves propagating from source 47 toward photosensor 48 by vane 37 signifies I an alarm, detection means 47 and 48 being formed and relatively positioned for the interruption of the propagating wave when vane 37 rotates sufficiently far in a direction indicated by arrow 49, the alarm position.

Once the vane has come to the armed position as above described, any intrusion whatsoever into enclosed volume 12 caused by an opening in structure 10 will be detected. There being no openings of sufficient size to allow access of the body of an intruder, the intruder must create an opening in structure 10 (for instance by forcing open a door) or must increase the size of an already existing opening (by further opening a window already partially open for ventilation purposes). The increased opening in enclosure 10 will produce a greater communication of enclosed volume 12 with external volume 13 preventing generating means 11 from maintaining the pressure differential required to displace vane 37 to its armed position as shown in FIG. 2. This pressure differential no longer being present across vane 37, the vane will fall under gravity biasing in the direction indicated by arrow 49 toward its equilibrium position 46. Prior to reaching equilibrium position 46, vane 37 will block the propagation of light from source 47 to photosensor 48, thus activating the alarm through the circuitry of FIG. 4, as will be explained below.

As will be immediately apparent the intrusion detection apparatus of the present invention can also be constructed with light source 47 and sensor 48 positioned so that vane 37 interrupts the beam of light or light waves in armed position 46 and allows transmission to sensor 48 in the alarm position. This will be accomplished if blower 11 is pulling air into volume 12 since vane 37 will be displaced downwardly. In such an approach vane 37 should be weighted so that the solid line position of FIG. 2 is the equilibrium alarm position, with center of gravity 44 directly under center of rotation 43, and position 46 is the anned position. This result can also be accomplished by moving source 47 so that it is shielded by vane 37 in the armed position and moving sensor 48 so that it sees" the light when the vane moves to position 46.

It should be emphasized that the intrusion detection apparatus of p the present invention is fully operable within multiple-room enclosures and is unaffected by openings and closings of internal doors, etc., within the enclosure. The only requirement regarding internal doors, etc., is that they maintain some small degree of communication between the rooms which they serve. Usually the typical space between the bottom of internal doors and the floor is sufficient to provide this communication. If in a rare case it is empirically established that the intrusion apparatus does not function for the entire volume enclosed by structure 10 with internal doors closed, it is only necessary to provide small vents in the doors to establish the required degree of pressure communication between the rooms. Given the internal communication above described, it has been found advantageous to mount generating means 11 as far away from sensing means 26 as possible. Such a configuration has been shown in FIG. 1.

Since opening of internal doors will cause shock waves that will momentarily effect the balanced vane sensor of the present invention, it is desirable to control the motion of vane 37 to provide pneumatic damping which prevents premature alarms while still maintaining the sensitivity of sensing means 26 to openings to the exterior of structure 10. The wall of housing 28 is formed to define with vane 37, at the distal ends thereof, passageways 51 and 52 for the flow of fluid past the ends of the vane; the passageways thus formed at either end of vane 37 being preferably of substantially constant dimension throughout the angular displacement of vane 37 between the armed position and the alarm position. This construction not only provides pneumatic damping but further dictates that the dynamic response of the vane will not vary as a function of agular displacement. The dimensions of passageways 51 and 52 are chosen such that a pressure wave deriving from opening or closing of internal doors (typically of approximately one-tenth of a second duration) will not cause the vane to swing to the alarm position. Instead, the vane will momentarily respond to the pressure wave and will be impeded due to the damping caused by the rather large volumes of fluid in chamber 29 and cavity 33 in comparison to the small areas in fluid escape 51 and 52. The vane thus cannot respond to internal phenomena fast enough to cause a false alarm. However, passageways 51 and 52 are large enough that the vane can respond to the fastest opening, entry and closing of an external door or window (typically approximately one second in duration).

As a further convenience adjustment means, generally designated 53, is provided mounted to housing 28 for selective adjustment of the response sensitivity of vane 37. Adjustment means 53 comprises bore 54 formed in housing 28 and extending between chamber 29 and enclosed volume 12, and variable pressure relief control means 56. Bore 54 and control means 56 cofunction to adjust the pressure differential acting across vane 37 by providing communication, in addition to that provided by passageway 51, of chamber 29 with enclosed volume 12. Since different enclosed volumes have differing leakages to the outside, the same pressure differential generating means will cause differing pressure differentials within different structures. Furthermore, occupants of structures in which the present invention may be used may differ in their desire for ventilation from the exterior while having intrusion detection apparatus of the present invention in an operative condition. Adjustment means 53 allows for these variations by enabling installation personnel to adjust the pressure differential across vane 37 and hence the response sensitivity of the vane. In practice, the enclosure is put into the condition of greatest desired communication with the exterior when the enclosure is to be protected and then pressure relief control means 56 is adjusted to cause vane 37 to come to its armed position when the enclosure is in this state. For leaky structures shutter 56 is closed, and for relatively tight structures shutter 56 can be opened.

Stop means 57 is further provided integrally with housing 28 and positioned to engage vane 37 for limiting the maximum angular displacement of the vane between the armed position and the alarm position. It is desirable to limit the angular distance through which vane 37 must travel in order that the vane will not require too long a time to swing to the alarm position upon the entrance into the enclosure of an intruder. As above noted, the vane must not require significantly longer than one second to swing from the armed position to the alarm position.

As an additional convenience wind baffle and filter means 58 can be provided to co-function with sensing means 26. Wind baffle and filter means 58 is mounted to communicate external volume 13 with chamber 29 through bore 34. The addition of means 58 permits greater regulation of the pressure differential between external volume 13 and enclosed volume 12. The effects of wind currents directly on vane 37 through unprotected bore 34 are nearly entirely eliminated and the filtering assists in keeping sensing means 26 clean and freely operable. Means 58 reduces turbulence through partial isolation of external volume 13 from enclosed volume 12 and is further preferably formed with considerable longitudinal extension to force any fluid flowing from the outside to the inside to be as constrained and controlled as possible before reaching chamber 29.

Alarm energizing circuitry generally designated 61 is shown in FIG. 4. The circuit is constructed so that an alarm connected to circuitry 61 through a relay generally designated 62 will normally be energized only when vane 37 has been first displaced to the alarm position. Relay 62 is energized by circuitry 61 when the light beam deriving from light source 47 is broken in its transit to phototransistor 48.

In order to provide the armed-before-alarm condition, circuitry 61 dictates that the optical connection must exist between source 47 and phototransistor 48 before the circuit is capable of energizing relay 62. Phototransistor 48 acts as a switch supplying voltage to the unijunction transistor generally designated 63 and its associated circuitry. When phototransistor 48 receives light, it supplies almost the entire available voltage to unijunction transistor 63. When there is no light being received at phototransistor 48, there is, for all practical purposes, no voltage on unijunction transistor 63. The key to this armed-before-alarm condition is diode 64 and capacitor 66 attached to the emitter of unijunction transistor 63. When voltage is applied to the unijunction transistor, capacitor 66 charges through a voltage divider comprised of resistors 67 and 68 to approximately 55 percent of the voltage across the unijunction transistor. Capacitor66 is clamped at this voltage by diode 64. Thereafter, when voltage is removed from unijunction transistor 63 through interruption of light propagating from source 47 to phototransistor 48, the voltage across the unijunction transistor falls to some point where the unijunction allows capacitor 66 to discharge across resistor 69. This voltage is between 2 and 3 volts, more than enough to activate silicon-controlled rectifier 71.

Even though unijunction transistor 63 may immediately reset itself, silicon-controlled rectifier 71 must be physically reset via reset switch 72. This dictates that once relay 62 is energized, it is latched on and hence an alarm connected to relay 62 will sound indefinitely awaiting resetting by means of switch 72.

The remaining circuitry is a simple power supply preferably with an output of 6.3 volts direct current. Typical values for the capacitors, the resistors and the transformer of circuitry 61 are listed below:

Capacitor C, 50 1f Capacitor C 50 ;1.f

Capacitor 66 5 pt" Resistor R 150 O Resistor 67 680 .Q

Resistor 68 I000 O Resistor 69 620 Q Transformer T, 6.3 volts The advantages of circuitry 61, in addition to the providing of the armed-before-alarm feature, are that it requires a small number of component parts and that it requires almost no regulation. The basic logic is dependent upon the ratio of the voltages between base 74 and emitter 73 of unijunction transistor 63.

We claim:

1. In intrusion detection apparatus for operation within a substantially enclosed volume, said apparatus having a pressure differential generating means mounted to communicate between and formed to establish a pressure differential between said enclosed volume and an external volume, and pressure differential sensing means mounted to sense said pressure differential, the improvement comprising:

said sensing means being formed with:

i. a housing defining a chamber formed for communication of said pressure differential between said enclosed and said external volumes therethrough;

ii. mounting means mounted relative to said housing to enable positioning of a vane therein;

iii. a vane pivotally mounted to and gravitationally balanced about said mounting means, said vane having a first vane portion extending across said chamber to provide a movable wall responsive to said pressure differential for pivotal movement to an armed position, and having a second vane portion positioned substantially outside said chamber, said vane and mounting means being formed to gravity bias said vane to an alarm position upon a change in said pressure differential; and

iv. detecting means mounted to sense the presence of said vane in said alarm position. 2. An intrusion detection apparatus as defined in claim ll wherein,

said first vane portion defines with a wall of said chamber a passageway for flow of fluid past said first vane portion and through said chamber, and said vane and said mounting means are formed to offset the axis of rotation of said vane from the center of gravity of said vane to effect gravity biasing of said vane upon angular displacement thereof from said alarm position.

3. An intrusion detection apparatus as defined in claim 2 wherein,

said first vane portion is gravitationally balanced to an alarm position below a horizontal plane passing through said mounting means and in said armed position is disposed above said horizontal plane. 4. An intrusion detection apparatus as defined in claim 2 wherein,

said wall of said chamber is formed to cause said passageway to have a substantially constant dimension throughout the angular displacement of said vane between said armed position and said alarm position. 5. An intrusion detection apparatus as defined in claim 4 wherein,

said housing is further formed to define a damping cavity formed for movement of said second vane portion therein, said damping cavity being defined by a housing wall formed to define with said second vane portion a second passageway having a substantially constant dimension throughout the angular displacement of said vane betweensaid armed position and said alarm position to assist in pneumatic damping of said sensing means. 6. An intrusion detection apparatus as defined in claim 4 wherein,

said housing is further formed with stop means positioned to engage said vane and limit the maximum angular displacement between said armed position and said alarm position. 7. An intrusion detection apparatus as defined in claim 1 wherein,

said detection means is formed to determine the presence of said vane in alarm position without physically contacting a movable portion of said vane and mounting means. 8. An intrusion detection apparatus as defined in claim 7 wherein,

said detecting means comprises a source propagating an electromagnetic wave, and electromagnetic wave detection means, said source and detection means being formed and relatively positioned for interruption of said wave by said vane when said vane is in one of said armed position and said alarm position. 9. An intrusion detection apparatus as defined in claim 8 wherein,

said source comprises a light emitting device; said electromagnetic wave detection means comprises a photo-sensor; and said vane interrupts the light from said light emitting device in said alarm position. 10. An intrusion detection apparatus as defined in claim 1 including,

adjustment means mounted to said housing and formed for selective adjustment of the response sensitivity of said vane. 11. An intrusion detection apparatus as defined in claim 10 wherein,

said adjustment means is comprised of a bore formed in said housing extending between said chamber and said enclosed volume in a position to adjust said pressure differential causing movement of said vane, and a variable pressure relief control means for adjusting said pressure differential across said vane. 12. An intrusion detection apparatus as defined in claim 1 wherein,

said sensing means is further formed with wind baffle and filter means mounted to communicate with said chamber and formed to reduce the effect of ume, and wherein,

said housing is formed to define a substantially cylindrical cavity having a medially disposed partition extending down the longitudinal axis of said cylindrical cavity to define said chamber on one side thereof and a damping cavity on the other side thereof pneumatically insulated from said chamber, and said housing defining said chamber is formed with a bore connected to said external volume for fluid flow into said chamber;

said vane is pivotally mounted to said housing and said first and second vane portions extend respectively across said chamber and said damping cavity and are dimensioned to extend from both sides of said partition to substantially the same distance therefrom and short of said housing to define passageways adjacent the distal ends of said first and second vane portions for fluid flow past said vane, said vane being further formed for pivotal support on said housing at a point above the center of gravity of said vane to gravitationally balance said vane and effect biasing of said vane to said alarm position; and

said detection means is comprised of a light emitting device and a photo-sensor mounted to cause said vane to interrupt light directed toward said photosensor in said alarm position and allow light to pass to said photo-sensor in said armed position. 

1. In intrusion detection apparatus for operation within a substantially enclosed volume, said apparatus having a pressure differential generating means mounted to communicate between and formed to establish a pressure differential between said enclosed volume and an external volume, and pressure differential sensing means mounted to sense said pressure differential, the improvement comprising: said sensing means being formed with: i. a housing defining a chamber formed for communication of said pressure differential between said enclosed and said external volumes therethrough; ii. mounting means mounted relative to said housing to enable positioning of a vane therein; iii. a vane pivotally mounted to and gravitationally balanced about said mounting means, said vane having a first vane portion extending across said chamber to provide a movable wall responsive to said pressure differential for pivotal movement to an armed position, and having a second vane portion positioned substantially outside said chamber, said vane and mounting means being formed to gravity bias said vane to an alarm position upon a change in said pressure differential; and iv. detecting means mounted to sense the presence of said vane in said alarm position.
 2. An intrusion detection apparatus as defined in claim 1 wherein, said first vane portion defines with a wall of said chamber a passageway for flow of fluid past said first vane portion and through said chamber, and said vane and said mounting means are formed to offset the axis of rotation of said vane from the center of gravity of said vane to effect gravity biasing of said vane upon angular displacement thereof from said alarm position.
 3. An intrusion detection apparatus as defined in claim 2 wherein, said first vane portion is gravitationally balanced to an alarm position below a horizontal plane passing through said mounting means and in said armed position is disposed above said horizontal plane.
 4. An intrusion detection apparatus as defined in claim 2 wherein, said wall of said chamber is formed to cause said passageway to have a substantially constant dimension throughout the angular displacement of said vaNe between said armed position and said alarm position.
 5. An intrusion detection apparatus as defined in claim 4 wherein, said housing is further formed to define a damping cavity formed for movement of said second vane portion therein, said damping cavity being defined by a housing wall formed to define with said second vane portion a second passageway having a substantially constant dimension throughout the angular displacement of said vane between said armed position and said alarm position to assist in pneumatic damping of said sensing means.
 6. An intrusion detection apparatus as defined in claim 4 wherein, said housing is further formed with stop means positioned to engage said vane and limit the maximum angular displacement between said armed position and said alarm position.
 7. An intrusion detection apparatus as defined in claim 1 wherein, said detection means is formed to determine the presence of said vane in alarm position without physically contacting a movable portion of said vane and mounting means.
 8. An intrusion detection apparatus as defined in claim 7 wherein, said detecting means comprises a source propagating an electromagnetic wave, and electromagnetic wave detection means, said source and detection means being formed and relatively positioned for interruption of said wave by said vane when said vane is in one of said armed position and said alarm position.
 9. An intrusion detection apparatus as defined in claim 8 wherein, said source comprises a light emitting device; said electromagnetic wave detection means comprises a photo-sensor; and said vane interrupts the light from said light emitting device in said alarm position.
 10. An intrusion detection apparatus as defined in claim 1 including, adjustment means mounted to said housing and formed for selective adjustment of the response sensitivity of said vane.
 11. An intrusion detection apparatus as defined in claim 10 wherein, said adjustment means is comprised of a bore formed in said housing extending between said chamber and said enclosed volume in a position to adjust said pressure differential causing movement of said vane, and a variable pressure relief control means for adjusting said pressure differential across said vane.
 12. An intrusion detection apparatus as defined in claim 1 wherein, said sensing means is further formed with wind baffle and filter means mounted to communicate with said chamber and formed to reduce the effect of wind currents in said external volume on said chamber.
 13. An intrusion detection apparatus as defined in claim 1 wherein, said detecting means includes alarm energizing circuitry constructed so that an alarm connected thereto will normally be energized only when said vane has been first displaced to said armed position then moved to said alarm position.
 14. An intrusion detection apparatus as defined in claim 1 having said pressure differential generating means mounted remote from said sensing means with said pressure differential generating means being formed to reduce the pressure inside said enclosed volume, and wherein, said housing is formed to define a substantially cylindrical cavity having a medially disposed partition extending down the longitudinal axis of said cylindrical cavity to define said chamber on one side thereof and a damping cavity on the other side thereof pneumatically insulated from said chamber, and said housing defining said chamber is formed with a bore connected to said external volume for fluid flow into said chamber; said vane is pivotally mounted to said housing and said first and second vane portions extend respectively across said chamber and said damping cavity and are dimensioned to extend from both sides of said partition to substantially the same distance therefrom and short of said housing to define passageways adjacent the distal ends of said first and second vane portions for fluid flow past said vane, said vane being furtHer formed for pivotal support on said housing at a point above the center of gravity of said vane to gravitationally balance said vane and effect biasing of said vane to said alarm position; and said detection means is comprised of a light emitting device and a photo-sensor mounted to cause said vane to interrupt light directed toward said photo-sensor in said alarm position and allow light to pass to said photo-sensor in said armed position. 